Photoresist

SEM images of the morphologies of meso structures and nanopatterns on (a) a positively nanopatterned silicon mold, and (b) a negatively nanopatterned silicon mold.

IntroductionPhotoresist : A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.Type: Positive: exposed regions dissolve (best resolution) Negative: Unexposed regions dissolve ( Swelling)

Structure: Resin: a binder that provides mechanical properties (adhesion, chemical resistance) Solvent: used to dissolve the resin, allowing the resin to be applied in a liquid state Photoactive Compound (PAC): Act to inhibit or promote the dissolution of the resin in the developer. PAC inhibits dissolution in positive resists before light exposure. After exposure the PAC promotes dissolution of the resin.

Chemistry

Positive Photoresist

Two-component DQN resists: DQN, corresponding to the photo-active compound, diazoquinone (DQ) and resin, novolac (N) Dominant for G-line (436nm) and I- line (365nm) exposure and not suitable for very short wavelength exposures Novolac (N): a polymer whose monomer is an aromatic ring with two methyl groups and an OH group.

dissolves in an aqueous solution easily Diazoquinone(DQ)

20-50 % weight Photosensitive DQ

UVCarboxylic acid ( dissolution enhancer)

Chemistry Positive Photoresist 1. DQ molecule will not dissolve in a base developer solution (pH >7).

2. UV light breaks the nitrogen molecule off forming an unstable molecule

2. To “stabilize” itself, one of the 6 carbon atoms in the ring pops out of the ring (leaving 5)

3. Once exposed to water (a developer /water mixture), an OH group attaches to the carbon atom, forming an acid.

4. The acid can then react and dissolve with the basic developer solution.

Advantage: Unexposed areas unchanged by the presence of the developer, line width and shape of a pattern precisely retained. Novolac fairly resistant chemical attack, a good mask for the subsequent plasma etching

Photoresist material parameters

a) Optical properties - resolution, photosensitivity, and index of refraction

b) Mechanical/chemical properties - solids content, viscosity, adhesion, etch resistance, thermal stability, flow characteristics, and sensitivity to ambient gases

c) Processing and safety related properties - including, cleanliness (particle count), metals content, process latitude, and shelf life.

Resolution - specifies the consistent ability to print minimum size images under conditions of reasonable manufacturing variation.The resolution of a lithographic process can be limited by many aspects of the process, including:a) Hardware (e.g. diffraction of light, lens aberrations)b) Optical properties of the resist material;c) Process characteristics (e.g. softbake step, develop step, etching step).

Resolution (µm) - linearity/ minimum Sensitivity (mJ/cm2) Focus margin (µm) Exposure margin (%) Dry etch resistance(X) Heat resistance Adhesion Standing wave effect (and bulk effect)(µm) BARC (bottom anti-reflective coating) compatibility Process margin/stability Shelf-life

Performance of Photoresists

Photoresists Profile

* Han Ku Cho, Samsung Electronics Co., Ltd, “Lithography technology review of what it is and what to be”, March 2003

Deep UV Photoresist

Limitation of Novolac based Photoresist

Strongly absorb below 250nm, KrF (248nm) marginally acceptable but not ArF (193nm)

Photoresist Solution for Submicron Features PMMA PAGs ( Photoacid generator) replace PAC Contrast enhancement layers (CELs) Inorganic resist (Ag-doped Ge-Se) Silicon-containing resists (dry developable) Multi-layer photoresist

PMMA (Polymethyl methacrylate) Short-wavelength lithography: deep UV, extreme UV, electron-beam lithography Resin itself is photosensitive

Advantage: high resolution

Disadvantage: Plasma etch tolerance is very low and thick PMMA to protect the thin film

Dissociation changes chemistry of the plasma etch and polymeric deposits on the surface of the substrate. Low sensitivity: Add PAG (chemically reactive dissociating) or elevate exposure

temperature*Stephen A. Campbell, “ The Science and Engineering of Microelectronic Fabrication”.

Contrast Enhance Layers (CELs)

CEM photo-bleachable Spun onto the DQN PR after softbake Formed in-situ “conformal contact mask” Enhanced contrast Important for DUV resists with less optical intense and PR radiation absorption

* http://www.microsi.com/photolithography/data_sheets/CEM%20365iS%20Data%20Sheet%

Inorganic Resist

Advantage:

High contrast γ ≈ 7

Produce fine line

Process: Ag-doped Ge-Se Ag plated on sputtered Ge-Se Photodoping create Ag2Se after exposure Dissolved in alkaline solution

Disadvantage: Require thick planarizing underlayer due to thin film nature Pineholes and defects from Ge-Se

*Stephen A. Campbell, “ The Science and Engineering of Microelectronic Fabrication”.

Dry developable: Polysilynes

Bi-layer process Silicon-containing resists on top of novolac based resist Highly resistant to plasma process Bleaching under DUV exposure due to cross-linked siloxane network Etch silicon selectively to silicon dioxide in HBr plasma

* Roderick R. Kunz, et al, “193 nm Resists and Lithography”, Polymers for Advanced Technologies, Volume 5, p p.12-21

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